1. Field of the Invention
The present invention relates to a vibration gyro for detecting an angular velocity.
2. Description of the Prior Art
Conventionally, mechanical rotary gyroscopes have been used as inertial navigation systems of airplanes and ships. The systems have been large in size and expensive. Thus, it has been difficult to build the gyroscopes into small electronic equipment and small conveying machines.
However, in recent years, miniaturization of gyroscopes has been studied to put a vibration gyro into practical use. In the vibration gyro, a vibrator is excited by a piezoelectric element, and voltage generated by vibration resulting from Coriolis force applied to the vibrator when it rotates is detected by another piezoelectric element provided on the vibrator. Such gyroscopes have been used for navigation systems of vehicles, shake detectors of video cameras, and so on.
Particularly, a vibration gyro using a piezoelectric single crystal is promising because the single crystal has a simple configuration, is adjusted with ease, and is excellent in temperature characteristics. As an example using the piezoelectric single crystal, the following will discuss the configuration and function of a tuning-fork vibration gyro using quartz in accordance with FIGS. 5 and 6.
The tuning-fork vibration gyro is formed by evaporating driving detecting electrodes onto a vibrator J10, on which quartz is integrally worked. The vibrator J10 is configured such that two tines J11 and J12 disposed laterally in parallel are connected to a base J15. Driving electrodes J1 to J4 are deposited onto the four sides of the left tine J11. Detecting electrodes J5 to J8 are deposited onto the four sides of the right tine J12. The bottom of the base J15 is used to support the vibration gyro.
Here, the extending direction of the tines J11 and J12 is referred to as a Yxe2x80x2-axis direction, the aligning direction of the tines J11 and J12 is referred to as an X-axis direction, and a direction orthogonal to X-axis and Yxe2x80x2-axis directions is referred to as Zxe2x80x2-axis direction. As shown in FIG. 5, a rectangular Cartesian coordinate of X-Yxe2x80x2-Zxe2x80x2 is formed by rotating the rectangular Cartesian coordinate of X-Y-Z, on which the X-axis and Z-axis conform to crystal axes, by xcex8 around the X-axis.
First, when the first tine J11 is bent to the second tine J12 in the X-axis direction, a part around an electrode J2 expands in the Yxe2x80x2-axis direction, and a part around an electrode J4 shrinks in the Yxe2x80x2-axis direction. At this moment, in the quartz, an electric field appears on the part around the electrode J2 in the X-axis direction and an electric field appears on the part around the electrode J4 in the xe2x88x92X-axis direction due to the piezoelectric effect.
At this moment, in view of the direction of the electric field, the electrodes J2 and J4 are equal in potential and are higher in potential than the center of the tines. In the X-axis direction, the electrodes J1 and J3 positioned near the center of the tines are relatively lower in potential than the electrodes J2 and J4. Thus, a potential difference appears between the electrodes J2 and J4 and the electrodes J1 and J3.
As the piezoelectric effect is reversible, when a potential difference is provided between the electrodes J2 and J4 and the electrodes J1 and J3, an electric field appears accordingly in the quartz, and the left tine J11 is bent in the X-axis direction.
Thus, the potentials of the electrodes J1 and J3 are amplified by an amplifier JG according to an amplification factor exceeding the oscillating condition, the phase is regulated by a phase-shift circuit JP so as to satisfy an oscillating condition, and the potentials are returned to the electrodes J2 and J4. Hence, energy is converted between mechanical return force, which is generated by the bending of the left tine J11, and electrical force, and the left tine J11 can be subjected to self-excited oscillation in the X-axis direction.
Entirely on the tuning-fork vibrator J10, in order to balance momentum between the left tine J11 and the right tine J12, when the left tine J11 is moved in the X-axis direction, the right tine J12 moves in the xe2x88x92X-axis direction, and when the left tine J11 moves in the xe2x88x92X-axis direction, the right tine J12 moves in the X-axis direction. The movements of the left and right tines J11 and J12 are called in-plane bending vibration, considering the fact that vibration in a single plane is generally regarded ideal for an ordinary tuning-fork. The vibrations of the first tine J11 caused by the amplifier JG and the phase-shift circuit JP are the same as the in-plane bending vibration. The frequency is substantially equal to a resonance frequency of the in-plane bending vibration of the vibrator J10.
In this state, when the vibrator J10 is entirely rotated around the Yxe2x80x2-axis with an angular velocity xcfx89, Coriolis force Fc is applied to the left and right tines J11 and J12 of the vibrator J10 in the Zxe2x80x2-axis direction, which intersects in-plane bending vibration. The Coriolis force Fc can be expressed by the equation below.
Fc=2xc2x7Mxc2x7xcfx89xc2x7V 
In this equation, M represents a mass of the left tine J11 or the right tine J12, and V represents a speed of the left tine J11 or the right tine J12.
The Coriolis force Fc excites bending vibration on the left tine J11 and the right tine J12. The bending vibration is displaced in the Zxe2x80x2-axis direction (orthogonal to the X-axis direction which is the operating direction of the in-plane bending vibration). Hereinafter, the bending vibration will be referred to as out-of-plane bending vibration. Further, Coriolis force does not increase in proportion to the displacement but to the speed. Thus, out-of-plane bending vibration generated by Coriolis force occurs with a phase delayed by 90xc2x0 from the in-plane bending vibration.
Due to the out-of-plane bending vibration, a part around electrodes J5 and J8 of the right tine J12 expands and shrinks in the Yxe2x80x2-axis direction, and a part around electrodes J6 and J7 expands and shrinks in opposite phase from the part around the electrodes J5 and J8.
For example, when the part around the electrodes J5 and J8 extends in the Yxe2x80x2-axis direction, an electric field appears in the X-axis direction on the part around the electrodes J5 and J8 in the right tine J12. At this moment, as the part around the electrodes J6 and J7 shrinks in the Yxe2x80x2-axis direction, an electric field appears in the xe2x88x92X-axis direction on the part around the inner electrodes J6 and J7 in the right tine 12. Namely, when the electrode J5 is higher in potential than the electrode J8, the electrode J7 is higher in potential than the electrode J6.
Moreover, when the part around the electrodes J5 and J8 shrinks in the Yxe2x80x2-axis direction, an electric field appears in the xe2x88x92X-axis direction on the part around the inner electrodes J5 and J8 in the right tine J12. At this moment, as the part around the electrodes J6 and J7 expands in the Yxe2x80x2-axis direction, an electric field appears in the X-axis direction on the part around the inner electrodes J6 and J7 in the right tine 12. Namely, when the electrode J5 is lower in potential than the electrode J8, the electrode J7 is lower in potential than the electrode J6.
A potential difference between the electrodes J5 and J8 and the electrodes J6 and J7 is changed according to the direction of the second tine J12 which vibrates in the Zxe2x80x2-axis direction. From a different point of view, when the electrode J5 has a high potential, the electrode J7 also has a high potential. At this moment, the electrodes J6 and J8 have low potentials. Meanwhile, when the electrode J5 has a low potential, the electrode J7 also has a low potential. At this moment, the electrodes J6 and J8 have high potentials. Coriolis force occurs as a potential difference between the electrode J5 or J7 and the electrode J6 or J8.
A detection signal of the Coriolis force is fed to one of the input terminals of a multiplying circuit JM via a differential buffer JD, which has the electrodes J5 and J7 as one input signal and the electrodes J6 and J8 as the other input signal. Further, the output of an oscillation system of in-plane bending vibration is fed to the other input terminal of the multiplying circuit JM via the amplifier JG, a phase-shift circuit JP2, and a comparator JC. The phase-shift circuit JP2 shifts the phase of the output of the amplifier JG by 90xc2x0 in order to correct Coriolis force which occurs with a delay of 90 degrees from the output of the oscillation system of in-plane bending vibration. The comparator JC binarizes the output of the phase-shift circuit JP2 to produce a reference signal.
The result of the multiplication and detection in the multiplying circuit JM is further smoothed by an integrating circuit JS and is detected as direct current output. The direct current output is in proportion to the Coriolis force Fc. Incidentally, as described above, as the Coriolis force Fc increases in proportion to the angular velocity xcfx89, the angular velocity xcfx89 can be found based on the direct current output.
However, the tuning-fork vibration gyro using a conventional piezoelectric single crystal has the following problems:
In general, when supporting a vibrator, in order to minimize the influence of the supporting effect on the vibrator, it is ideal to support the vibrator at a position where it hardly moves during vibration, that is, only at a node of vibration. The tuning-fork vibration gyro handles two-way bending vibration orthogonal to the extending direction of tines. In the two-way bending vibration, regarding in-plane bending vibration used for driving, an ideal support can be substantially realized by supporting the bottom of a base. In this supporting method, the tuning-fork vibrator only slightly vibrates in the extending direction of the tines, and the frequency is changed by several PPM in accordance with a change on a supporting part.
Meanwhile, for a tuning-fork vibrator, out-of-plane bending vibration excited by Corirolis force, which occurs on the tines due to in-plane bending vibration and the rotation of the vibrator, turns into a torsional vibration around a center symmetry axis of the tuning fork. Thus, it is difficult to support the vibrator without transmitting vibrations to the outside.
When supporting a vibrator in such a conventional manner as exerts a disadvantageous effect on the vibrator, such as supporting it at the bottom of the base, out-of-plane bending vibration, which is detecting vibration generated by Coriolis force, and leakage vibration of in-plane bending vibration which is driving vibration described later to detecting vibration, are conveyed to the outside of the vibrator via the supporting part, resulting in a reduced S/N ratio and the occurrence of drift.
Further, in a tuning-fork vibration gyro with two tines, a stick-shaped vibrator performs both driving and detection and the detecting part vibrates. Theoretically, as the driving direction and the detecting direction intersect orthogonally each other, the driving vibration does not affect the detecting vibration. However, in the case of an actual working accuracy, the orthogonality is not enough so that vibration slightly occurs as leakage in the direction of the out-of-plane bending vibration. Hence, a detecting electrode detects the leakage vibration caused by the driving vibration.
If the resonance frequency of driving vibration is separated from the resonance frequency of detecting vibration, a slight excitation of detecting vibration by driving vibration can be reduced. However, in the vibration gyro, as the resonance frequencies of the driving vibration and the detecting vibration are close to each other to allow transmission of Coriolis force, it is not possible to prevent driving vibration from leaking to detecting vibration.
Further, due to electrostatic capacity coupling between a driving electrode and a detecting electrode, the detecting electrode detects driving vibration. This indicates that detecting output is produced despite the absence of Coriolis force. As the driving vibration has large amplitude, the detecting vibration of small amplitude is considerably affected by a change in environment of a supporting part and a slight change of the driving vibration that is caused by a change in temperature of the vibrator, resulting in a reduced S/N ratio and the occurrence of drift.
The vibration gyro of the present invention has a vibrator composed of a base and three tines which are aligned in a single line at prescribed intervals in one direction on the base and extending in a direction perpendicular to the aligning direction. Of the three tines, a central second tine and an adjacent first tine on the right or left are driven by oscillator. Further, Coriolis force generated on the other third tine is detected by detector.
The following aspects are applicable in the present invention.
In the vibration gyro, when the first and second tines are driven by the oscillator, the dimensions of the individual parts are determined so as to cause the third tine to substantially stand still.
In order to cause the third tine to substantially stand still when the first and second tines of the vibrator are driven by the oscillator, the following aspects (1) and (2) are applicable.
(1) A width W3 of the third tine is smaller than a width W1 of the first tine and a width W2 of the second tine ((WI, W2) greater than W3).
Moreover, in this case,
the width of the first tine is equal to the width W2 of the second tine (W1=W2), and
the width W3 of the third tine is reduced by 10 to 20% from the width W1 of the first tine and the width W2 of the second tine (W3=0.8 to 0.9xc3x97(W1, W2)).
(2) A step (shoulder) is formed between the side of the first tine that is opposite from the second tine and the side of the base at the side of the first tine.
Moreover, in this case, the width W1 of the first tine and the width W2 of the second tine are equal to each other, and the width W3 of the third tine is ⅗xc2x110% of the width W1 of the first tine (W1=W2; W3=0.54 to 0.66xc3x97(W1, W2)).
Besides, in the vibrating gyro of the present invention, the first tine and the second tine are caused to make first bending vibration within a plane perpendicular to the thickness direction of the vibrator by using the oscillator. And then, second bending vibration perpendicular to the plane, which is caused on the vibrator by the first bending vibration due to Coriolis force resulting from the rotation of the vibrator, is detected by the detector using the third tine.